Silicone rubber compositions, their preparation and use

Abstract: A silicone rubber composition comprising (1) a diorganopolysiloxane blocked with a hydroxysilyl group at either end, (2) a silane or siloxane having at least three hydrolyzable silyl groups in a molecule, and (3) a divalent organic tin compound in a form substantially isolated from air quickly cures into a silicone rubber elastomer simply by heating at 40 to 200°C.

Description: This invention relates to silicone rubber compositions curable to silicone rubber elastomers having rubbery properties. Prior Art Prior art room temperature vulcanizable (RTV) silicone rubber compositions of the condensation type are generally based on a diorganopolysiloxane end-blocked with a hydrolyzable silyl group or based on a diorganopolysiloxane blocked with a hydroxysilyl group at either end and a silane or siloxane having at least three hydrolyzable silyl groups in a molecule. They are cured with the aid of curing catalysts such as tetravalent organic tin compounds and organic titanium compounds. The silicone rubber compositions based on a diorganopolysiloxane end-blocked with a hydrolyzable silyl group do not cure without water or humidity and are not susceptible to curing-promotion by heating. It is difficult to cure these compositions within a short time. The latter silicone rubber compositions based on a diorganopolysiloxane blocked with a hydroxysilyl group at either end and a silane or siloxane having at least three hydrolyzable silyl groups in a molecule cure fast and are less affected by water as compared with the former compositions. However, water supply is indispensable in order to allow heavy-gage moldings of these compositions to cure deep inside. To this end, water, alcohol or the like is added to the compositions. Even when such a technique is employed, large sized moldings require an undesirably long time until curing is completed. One known heat curing type of liquid silicone rubber composition is a composition of the addition curing type relying on hydroxysilylation reaction. The liquid silicone rubber compositions of heat curing type are advantageous in that the curing rate is increased by heating and the pot life is readily adjustable, but have a disadvantage associated with the use of platinum series compounds as the catalyst. When contacted with compounds having an ability to coordinate with a platinum series metal, for example, compounds or materials containing sulphur, amine or tin, the compositions are inhibited from curing by catalyst poisoning. Therefore, the compositions are applicable only under very carefully controlled conditions. When a diorganopolysiloxane blocked with a hydroxysilyl group at either end, a silane or siloxane having at least three hydrolysable silyl groups in a molecule, and a divalent organic tin compound, in a form substantially isolated from air, are blended one can obtain a silicone rubber composition which upon heating is quickly curable into a silicone rubber elastomer having excellent rubbery properties without being inhibited from curing. Divalent organic tin compounds used as a curing catalyst may allow a silicone rubber composition comprising a diorganopoly-siloxane blocked with a hydroxysilyl group at either end and a silane or siloxane having at least three hydrolysable silyl groups in a molecule to cure quickly, even deep inside, simply by heating at a temperature of about 40 to 200°C and without a need for water, producing a useful silicone rubber elastomer. The resulting silicone rubber elastomer or cured product can have good rubbery properties since depolymerisation or cracking of siloxane under high temperature enclosure is minimised as compared with the use of tetravalent organic tin catalysts. Although a silicone rubber composition having a divalent organic tin compound blended therein has a potential problem in that, even when divided into separate packages for storage, it ceases to cure after several days or a few months, its shelf stability is we find significantly improved by substantially isolating the divalent organic tin compound from air. This, coupled with freedom from cure inhibition by catalyst poison, allows such a composition to find a wide variety of applications. US-A-4461854 describes a variety of room-temperature vulcanisable two-component silicone rubber compositions using a hydroxy-terminated siloxane, a cross-linking agent (which may be a trialkoxy silane) and a heat-activated metal carboxylate catalyst such as a tin carboxylate; tin (II) compounds are included. The catalyst is encased in a heat-meltable room-temperature-inert barrier material such as calcium stearate, thereby improving the composition's working lifetime. The general aim herein is to provide new and useful silicone rubber compositions using divalent tin catalyst isolated from air, and methods in which they are prepared and used. Preferred aims included (a) the ability to achieve quick cure at the interior of heavy-gauge parts; (b) cure that is promoted simply by heating; (c) cure relatively non-susceptible to inhibition; (d) good rubbery properties in the resulting elastomer. In one aspect we provide a silicone rubber composition comprising diorganopolysiloxane end-blocked with hydroxysilyl groups; silane or siloxane having at least three hydrolysable silyl groups per molecule, and organic divalent tin compound in a form isolated from air, characterised in that the tin compound is isolated from air by inclusion in (a) silicone resin of the formula K 3 a SiO (4-a)/2 where R 3 is a substituted or unsubstituted monovalent hydrocarbon group having 1 to 10 carbon atoms and a is in the range from 0.8 to 1.8. Also contemplated herein is a method comprising curing a silicone rubber composition as defined above by heating the composition at a temperature in the range from 40 to 200°C, and certain methods for the preparation of the composition as set out in claims 12 and 13. A first component of the silicone rubber composition is diorganopolysiloxane end-blocked with hydroxysilyl group. Preferred are diorganopolysiloxanes of the following general formula (1). In formula (1), R are substituted or unsubstituted monovalent hydrocarbon groups having 1 to 10 carbon atoms, for example, methyl, ethyl, propyl, phenyl, vinyl and trifluoropropyl groups. If necessary, some R groups may have a hydroxysilyl substituent. Letter n is an integer of 100 to 1,000. The diorganopolysiloxanes of formula (1) preferably have a viscosity of 1-1000 x 10 -4 m 2 /S (100 to 100,000 centistokes) at 25°C. They may generally be synthesized by effecting equilibration reaction of organocyclopolysiloxanes in the presence of an alkali or acid catalyst using water or a silanol group-containing low molecular weight compound as a reaction stopper. Exemplary alkali catalysts are potassium hydroxide, tetraalkylphosphonium hydroxide, and tetraalkylammonium hydroxide and exemplary acid catalysts are sulfuric acid, methanesulphonic acid and trifluoromethanesulphonic acid. A second component is silane or siloxane having at least three hydrolysable silyl groups in a molecule. Since it serves as a cross-linking agent, it must contain in a molecule at least three hydrolysable functional groups capable of reacting with silanol groups in the diorganopolysiloxane (1) to form an elastomer. The silane or siloxane of the second component may be represented by the following general formula: R 1 a R 2 b SiO (4-a-b)/2 wherein 0.1 < a ≦ 4 and 2 < a + b ≦ 4. In the formula, R 1 is unsubstituted or substituted monovalent hydrocarbon group having 1 to 10 carbon atoms such as an alkyl group including methyl, ethyl, propyl, isopropyl, butyl and isobutyl, an alkenyl group e.g. vinyl, allyl and propenyl, an aryl group e.g. phenyl, and substituted groups in which some or all of hydrogen atoms in the above hydrocarbon groups are replaced by a halogen atom or a cyano group. R 2 is a hydrolysable functional group. Hydrolysable functional groups usable herein include acyloxy, ketoximito, amino, amido, acid amido, aminoxy, alkoxy, and alkenyloxy groups as is well known in the art. When the composition is intended for application to electric and electronic parts, useful groups are alkoxy, acyloxy, alkenyloxy and alkoxyalkenyloxy groups directly attached to a silicon atom. Examples of the silane include methyltrimethoxysilane, vinyltrimethoxysilane, phenyltrimethoxysilane, vinyltriethoxysilane, tetraethoxysilane, tetra(n - propoxy)silane, methyltris(propenoxy)silane, vinyltris (propenoxy)silane, phenyltris(1 - methoxy - 2-methylpropenoxy)silane, hydrolysates thereof, and mixtures thereof. Shown below are examples of the siloxane. In the formulae, Me is methyl and Et is ethyl. Preferably the second component, silane or siloxane, is blended in an amount of about 0.5 to about 20 parts, especially about 1 to about 10 parts by weight per 100 parts by weight of the first component, diorganopolysiloxane. Some compositions with less than 0.5 parts of the second component would not cure into an elastomer whereas compositions with more than 20 parts of the second component may cure to a product too hard to be acceptable as an elastomer. A third component is divalent organic tin compound serving as a catalyst. when such tin compounds are kept in air as such, their catalysis to promote curing is typically soon lost. Blending the tin compound in a form substantially isolated from air gives a shelf-stable composition. Examples of the divalent organic tin compound include tin octylate, tin naphthenate, and tin stearate. These organic tin compounds are isolated from air in a substantial sense. Suitable isolation measures include the following. The divalent organic tin compound or a carrier having the compound adsorbed thereto is mixed with a silicone resin having a melting point of 40 to 200°C and the mixture is then finely divided or spray dried to form catalyst particles in which the compound is substantially isolated from air. See JP-B-41707/78 for a relevant description (for platinum catalysts). More specifically, a silicone resin is melted under heat. To the melted silicone resin is added and mixed a divalent organic tin compound under an inert atmosphere such as nitrogen atmosphere. After cooling, the solid mixture is pulverized. Alternatively, silicone resin and divalent organic tin compound are dissolved into an organic solvent, followed by spray drying to evaporate the organic solvent and recover powders. The present composition may then be stored in a single package. If the silicone resin used as a barrier is a compound soluble in the second component, that is, the silane or siloxane having at least three hydrolysable silyl groups per molecule, then the present composition is readily curable at lower temperatures. Examples of the silicone resins are those represented by the following formula: R 3 a SiO (4-a)/2 wherein R 3 is an unsubstituted or substituted monovalent hydrocarbon group as for R 1 and a is a positive number of 0.8 to 1.8. Preferably, R is methyl group, phenyl group or fluorine - substituted alkyl group such as trifluoropropyl group, C 8 F 17 CH 2 CH 2 , etc. In the case of microcapsulation, microcapsules preferably have an average diameter of about 0.1 to 50 µm although the capsule diameter is not particularly limited. Various additives may be blended in the silicone rubber composition to improve the properties thereof. Exemplary additives are fumed silica and precipitated silica which has been or has not been treated to be hydrophobic, and carbon black as reinforcing fillers, anti-settling agents or agents for improving electric conductivity; ground quartz, fused quartz, spherical silica, diatomaceous earth, zeolite, calcium carbonate, titanium dioxide, iron oxide, alumina, spherical alumina, aluminum hydroxide, aluminum nitride, and magnesium sulfate as semi-reinforcing fillers, extenders or heat conducting fillers; and lead compounds such as lead, lead carbonate and lead hydroxide as radiation barriers. Also there may be blended various carbon functional silanes used for the purpose of imparting tackiness to condensation type silicone rubber compositions, for example, such as amino group-containing alkoxysilanes, epoxy group-containing alkoxysilanes, and mercapto group - containing alkoxysilanes. Further coloring agents such as inorganic pigments and organic dyes, heat resistance enhancers such as cerium oxide, zinc carbonate, manganese carbonate, benzotriazole, and platinum compounds, and flame retardancy enhancers may also be added insofar as the present curing properties are not lost. The silicone rubber composition may be stored stably in a single package containing all the components. Alternatively, the second component (silane or siloxane) and the third component (organic tin compound) are stored in two separate packages. One package contains the second component and another package contains the third component. The first component (diorganopolysiloxane) and other optional components may be admitted into both or either of the packages. Such divided blending may be suitably selected. A suitable curing method comprises mixing all the necessary components, optionally molding the mixture, and heating the mixture whereby it is cured (including any deep interior) to form a silicone rubber elastomer. In general, brief cure is obtained by heating at a temperature of about 40 to 200°C, especially about 60 to 150°C. Temperature lower than 40°C would be too low to provide a quick cure rate whereas temperatures higher than 200°C would cause deterioration of the silicone rubber itself. The heating time may be about 1 to 60 minutes. EXAMPLE Examples of the present concepts are given below by way of illustration and not by way of limitation. All parts are by weight. Reference Example 1 In a glass container, 100 grams of a silicone resin obtained by hydrolysis of 70 mol% of phenyltrichlorosilane, 25 mol% of methyltrichlorosilane, and 5 mol% of diphenyldichlorosilane and having a softening point of 80°C was dissolved in 500 grams of dichloromethane. With the solution was mixed 5 grams of tin octylate in a nitrogen atmosphere. This solution was atomized through a spray dryer which had been internally purged with nitrogen gas and had an inlet temperature of 90°C and an outlet temperature of 40°C, obtaining 70 grams of fine particles of the organic tin compound having an average particle diameter of 20 µm. Reference Example 2 In 500 grams of dichloromethane was dissolved 100 grams of the same silicone resin having a softening point of 80°C as used in Reference Example 1. With the solution was mixed 5 grams of dibutyltin dilaurate in a nitrogen atmosphere. As in Reference Example 1, the solution was atomized through a spray dryer, obtaining 68 grams of fine particles of the organic tin compound having an average particle diameter of 20 µm. Reference Example 3 In 500 grams of dichloromethane was dissolved 100 grams of polystyrene having a softening point of 80°C. With the solution was mixed 5 grams of tin octylate in a nitrogen atmosphere. As in Reference Example 1, the solution was atomized through a spray dryer, obtaining 70 grams of fine particles of the organic tin compound having an average particle diameter of 20 µm. The particles were washed with water and dried at 40°C in a vacuum of 10 mmHg. Examples (Examples 2, 5, 6-1, 6-2 and 7 are comparative examples) Silicone rubber composition 1: To 100 parts of a dimethylpolysiloxane end-blocked with a silanol group and having a viscosity of 5,000 centipoise at 25°C were added 2 parts of octaethoxytrisiloxane and 3 parts of the organic tin compound particles obtained in Reference Example 1. Thorough mixing gave a silicone rubber composition 1. The particles were finely dispersed in the mixture. Silicone rubber composition 2: A silicone rubber composition 2 was prepared by the same procedure as composition 1, but used 0.15 parts of tin octylate added directly instead of the organic tin compound particles obtained in Reference Example 1. The mixture was uniform. Silicone rubber composition 3: To 100 parts of a dimethylpolysiloxane end-blocked with a silanol group and having a viscosity of 5,000 centipoise at 25°C were added 2 parts of octaethoxytrisiloxane, 50 parts of heavy calcium carbonate, and 3 parts of the organic tin compound particles obtained in Reference Example 1. Thorough mixing gave a uniform silicone rubber composition 3. It was an apparently uniform white viscous mixture. Silicone rubber composition 4: To 100 parts of a dimethylpolysiloxane end-blocked with a silanol group and having a viscosity of 5,000 centipoise at 25°C was added 4 parts of octaethoxytrisiloxane. This mixture was placed in a package (A). To 100 parts of a dimethylpolysiloxane end-blocked with a silanol group and having a viscosity of 5,000 centipoise at 25°C was added 6 parts of the organic tin compound particles obtained in Reference Example 1. This mixture was placed in a package (B). A set of packages (A) and (B) gave a silicone rubber composition 4. On use, the contents of packages (A) and (B) were mixed in a weight ratio of 100:100. Silicone rubber composition 5: To 100 parts of a dimethylpolysiloxane end-blocked with a silanol group and having a viscosity of 5,000 centipoise at 25°C were added 2 parts of octaethoxytrisiloxane, 50 parts of heavy calcium carbonate, and 3 parts of the organic tin compound particles obtained in Reference Example 3. Thorough mixing gave a uniform silicone rubber composition 5. It was an apparently uniform white viscous mixture. Silicone rubber composition 6: A dimethylpolysiloxane end-blocked with a silanol group and having a viscosity of 5,000 centipoise at 25°C, 100 parts, was placed in a package (A). A mixture of 2 parts of octaethoxytrisiloxane and 0.15 parts of tin octylate was placed in a package (B). A set of packages (A) and (B) gave a silicone rubber composition 6. On use, the contents of packages (A) and (B) were mixed in a weight ratio of 100:2. This composition was designated composition 6 - 1 when package (B) was allowed to stand in air and composition 6 - 2 when package (B) was stored in nitrogen in a sealed glass bottle. Silicone rubber composition 7: This is a comparative example. To 100 parts of a dimethylpolysiloxane end - blocked with a silanol group and having a viscosity of 5,000 centipoise at 25°C were added 2 parts of octaethoxytrisiloxane, 50 parts of heavy calcium carbonate, and 3 parts of the organic tin compound particles obtained in Reference Example 2. Thorough mixing gave a uniform silicone rubber composition 7. It was an apparently uniform white viscous mixture. Each of the compositions was cast into an aluminum dish having a diameter of 6 cm and a depth of 1 cm and heated at 100°C for 30 minutes into a cured product, which was measured for hardness (JIS A scale). Similar measurements were done after the compositions were aged at room temperature for 3 days. The results are shown in Table 1. Composition No. 1 2 3 4 5 6 - 1 6 - 2 7 Hardness, fresh 16 16 48 15 10 18 18 Uncured Hardness, aged 16 - 45 14 8 uncured 15 uncured As is evident from Table 1, silicone rubber compositions having blended therein the divalent organic tin compound in a form substantially isolated from air quickly cured by heating, forming silicone rubber elastomers having excellent rubbery properties, and also retained that ability even after storage. There have been described silicone rubber compositions which, upon heating, quickly cure deep inside without cure inhibition. They form silicone rubber elastomers which will find a variety of applications because of excellent rubbery properties. Although some preferred embodiments have been described, many modifications and variations may be made thereto in the light of the above teachings.

Claims: A silicone composition heat-curable to a silicone rubber elastomer, comprising diorganopolysiloxane end-blocked with hydroxysilyl groups; silane or siloxane having at least three hydrolysable silyl groups per molecule, and organic divalent tin compound in a form isolated from air, characterised in that the tin compound is isolated from air by inclusion in silicone resin of the formula R 3 a SiO (4-a)/2 where R 3 is substituted or unsubstituted monovalent C 1-10 hydrocarbon group and a is in the range from 0.8 to 1.8. A composition according to claim 1 in which R 3 , when substituted, is selected from halo- and cyano-substituted C 1-10 monovalent hydrocarbon groups. A composition according to claim 2 in which R 3 is selected from methyl, phenyl and fluoro-substituted alkyl. A composition according to any one of claims 1 to 3 in which the silicone resin has a melting point in the range 40 to 200°C. A composition according to any one of the preceding claims in which the tin compound is microencapsulated. A composition according to any one of the preceding claims in which the tin compound is selected from tin octylate, tin naphthenate and tin stearate. A composition according to any one of the preceding claims in which said silane or siloxane is present at from 0.5 to 20 parts by weight per 100 parts by weight of said end-blocked diorganopolysiloxane. A composition according to any one of the preceding claims in which said silane or siloxane is of the formula R 1 a R 2 b SiO (4-a)/2 in which groups R 1 are selected from C 1-10 unsubstituted monovalent hydrocarbon groups and halo- or cyano-substituted C 1-10 monovalent hydrocarbon groups; R 2 are hydrolysable functional groups, 0.1 < a ≤ 4 and 2 < a+b ≤ 4. A composition according to claim 5 in which said silane is selected from methyltrimethoxysilane, vinyltrimethoxysilane, phenyltrimethoxysilane, vinyltriethoxysilane, tetraethoxysilane, tetra(n-propoxy)silane, methyltris(propenoxy)silane, vinyltris(propenoxy)silane, phenyltris(1-methoxy-2-methylpropenoxy)silane and hydrolysates and mixtures thereof. A composition according to claim 9 in which said siloxane is or wherein Me is methyl and Et is ethyl. A composition according to any one of claims 1 to 3 wherein said end-blocked diorganopolysiloxane is of the formula wherein groups R are selected from substituted and unsubstituted monovalent hydrocarbon groups having from 1 to 10 carbon atoms, optionally with hydroxysilyl substituent, and n is from 100 to 1000. A method of preparing a composition according to claim 4 in which the air-isolated tin compound is prepared by mixing the tin compound with the melted silicone resin under an inert atmosphere, cooling and pulverising. A method of preparing a composition according to claim 4 in which the air-isolated tin compound is prepared by dissolving it with the silicone resin in an organic solvent and spray drying to evaporate the solvent and produce a powder. A method of making a silicone rubber elastomer comprising heating a composition according to any one of claims 1 to 12 at a temperature in the range 40 to 200°C.

Inventors: FUJIKI HIRONAO, FUJIKI, HIRONAO
Applicants: SHINETSU CHEMICAL CO, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1994
Main classification: C08K5
IPC codes: C08K5, C08L83, C08J3, C08K9